This invention pertains to a valve which is designed for use with a fuel storage tank. Such tanks are conventionally attached to pumps which pump the fuels stored in the tank for delivery to a customer. If the tank is not vented to allow outside air to enter, a vacuum develops in the tank as fuel is withdrawn therefrom. As more and more fuel is pumped out of the tank, the pressure of the vacuum increases and thus the pump must operate against an ever-increasing load. Such a load results in a reduction in the service life of the pump and additionally results in a lessening of the rate of fuel delivery from the pump.
Thus, it is conventional practice to vent the storage tank to the atmosphere by means of a vent pipe which can allow air to enter the tank during the period when fuel is being withdrawn therefrom by the pump. However, the use of a vent pipe alone opens the interior of the tank to the atmosphere at all times.
Such fuels as gasoline have a high volatility, and thus gasoline in the storage tank can easily enter a low pressure vapor phase. In the event that a vent pipe alone is used to vent a storage tank which contains gasoline or another volatile fuel, the gasoline in the low pressure vapor phase will be free to leave the storage tank through the vent pipe. This phenomenon not only results in a loss of gasoline in the tank to the atmosphere, but also allows toxic hydrocarbons to pollute the atmosphere adjacent the vent pipe. This phenomenon is particularly acute where the storage tank is buried underground and a long vent pipe is used to vent the storage tank to the atmosphere. When such a long vent pipe is used, a high stack draft is developed which causes the gasoline in the low pressure vapor phase to be positively sucked out of the storage tank.
It would thus be desirable to provide a device which will allow the storage tank to be vented to the atmosphere during a withdrawal of fuel therefrom by a pump, but which would also prevent such venting in the event that the pump is not in use, to eliminate pollution problems and loss of fuel to the atmosphere.